The aim of the study was to determine the effect of fertilization with various doses of ash from biomass combustion (balanced to the amount of K2O introduced into the soil) on the growth of representatives of soil microorganisms, i.e., diatoms. In a one-factor field experiment (control, NPK, 100, 200, 300, 400, 500 kg/ha), soil samples were collected from the 0–5 cm layer of podzolic soil under the cultivation of winter oilseed rape (Brassica napus L. var. napus) at the end of August 2019 and 2020. The biomass combustion ash used for soil fertilization was characterized by an alkaline reaction (pH = 12.83 ± 0.68) and high levels of basic macroelements required for proper plant growth and development. The particle size distribution in each plot was identified as loamy silt (pgl). Before the experiment (autumn 2018), the soil exhibited an acidic reaction (pHH2O = 5.8), low conductivity (EC = 68 µS), and 19.09% moisture at the 0–5 cm level. In total, 23 diatom species were identified in the material collected from the topsoil in all variants of the experiment. Hantzschia amphioxys, Mayamaea atomus, Mayamaea permitis, Nitzschia pusilla, Pinnularia obscura, Pinnularia schoenfelderi, and Stauroneis thermicola were the most abundant populations.